The delivery pain (i.e., labor pain) is second in the pain rating index after burning pain. A severe labor pain may have a negative impact on the physical and mental health of the parturient and the newborn. Many parturients choose cesarean which leads to unnecessary injuries to the body, because they are unable to bear the severe labor pain, or even because they fear the labor pain. There are many reasons causing delivery pain, for example, the paroxysmal contraction of the uterine, the stretching or tearing of muscle fibers of the uterine, and the compression of the vessels of the uterine may trigger pain centers in the brain to make the parturient feel severe pain. When a fetus pass through the birth canal, the fetus may expand the birth canal, especially lower uterine segment, cervix, vagina and perineum, on which damage or traction may occur to cause pain. The nervousness, anxiety or fear of the parturient will cause neuroendocrine responses in the body to amplify the pain.
Due to individual differences, the tolerance and feeling of pain is different for everyone. According to some statistics, about 44 percent of primiparas may feel that the delivery pain is unbearable and even “be dying of pain”. This, however, may be correlated with psychogenic factors. Some primiparas feel the delivery pain is not as unbearable as imagine, and they may only feel the distension and pain of the abdomen and the lumbosacral region.
At present, the pain relief at childbirth has the following methods: (1) drug pain relief at childbirth (epidural block), having a reliable analgesic effect, but turning the labor from a physiological process to a pathological process, and needing a specific operation technique, which may affect the fetus and extend the labor, (2) nitrous oxide (i.e., N2O) inhalation, belonging to general anesthesia that the puerperant is required to be fasted, which may depress the cardiovascular system and the central nervous system of the fetus and the newborn; (3) Lamaze breathing, as a psychological analgesia belonging to a mental and psychological therapy; (4) free posture, Doula delivery; (5) transcutaneous electrical nerve stimulation (TENS) pain relief at childbirth, defined as a method for electrically stimulating the skin to reduce pain, by the clinical electrophysiology and American physiological society. The Gate-control theory of pain is one of the theoretical bases for TENS. TENS is a noninvasive way, has a good result with analgesia, does not have side effects on the maternal body and the fetus, and is one of the important non-pharmacological methods for pain relief at childbirth.
There have been many studies about the non-invasive pain relief at childbirth technique. A Chinese utility model patent CN 202086950U, with a publication date of 26 Sep. 2012, discloses a system for pain relief at childbirth, which determines the degree of pain of the parturient according to the signal from the measured blood pressure, sends pulse signals of different pulse widths and strength to stimulate the stomach to sooth the nerve system for analgesia, and plays music and video simultaneously to relieve nervous tension of the parturient to reduce the pain of the parturient. A Chinese utility model patent CN 202086950 U, with a publication date of 28 Dec. 2011, discloses a device for pain relief at childbirth, which combines transcutaneous electric nerve stimulation, pulse therapy and music therapy, while makes use of acupuncture and moxibustion therapy, to provide a green, safe and effective apparatus for pain relief at childbirth. A Chinese utility model patent CN 2737391Y, with a publication date of 29 Sep. 2014, discloses an apparatus for pain relief at childbirth, which generates a series of pulses according to the frequency given by the user, in which the strength of the pulses is adjusted, and the pulses are amplified to be output in isolation by a pulse converter, to act on specific sites of the body to activate the body's own analgesic substance, so as to achieve the purpose of analgesia. A Chinese utility model patent CN 1148989A, with a publication date of 7 May 1997, discloses a computer-based apparatus for pain relief at childbirth and monitoring, which includes an analgesia circuit, a fetal heart sound circuit, a uterine pressure circuit and a fetal movement circuit, simultaneously has the function of analgesia and monitoring, and can give an alarm with sound and light, in which the output of analgesia signals is divides into three channels including a pulse using pulse code modulation (300˜600 Hz), a low frequency pulse (2˜10 Hz) and a medium frequency pulse (2,000˜8,000 Hz), which can be selected by a manual button or automatically controlled based on the strength of uterine contraction. A Chinese utility model patent CN 2216389Y, with a publication date of 3 Jan. 1996, discloses an apparatus for pain relief at childbirth and monitoring with computer, including a control circuit for the strength of the output of the analgesia signals, a monitoring, recording and alarming system for fetal heart sound, and a monitoring and recording system for uterine contraction, which can control the strength of the output of the analgesia signals automatically or manually according to the uterine contraction. A United States utility patent U.S. Pat. No. 7,942,818 B2, with a publication date of 7 May 2011 discloses a method for predicting the time at which a delivery pain happens with signals of uterine contraction.
In the above patent documents related to methods and apparatus for pain relief at childbirth, the degree of the delivery pain is generally determined based on the strength of the uterine contraction, that is, the more strength of the uterine contraction, the greater the amplitude of the wave for stimulating is. And in the above patent documents related to methods and apparatus for pain relief at childbirth, the output wave is a simple pulse signal, and varies only based on the variation of the pulse width, frequency or strength, and there is a single output signal, so the effect of the analgesia is not ideal. In addition, the living body is easy to produce resistance for the single electrical-stimulation that acts on the body for a long time, such that the analgesic effect of the originally effective analgesic stimulation signal will be reduced or even failure after long time.